1. Field of the Invention
The present invention relates to a method of correcting an image signal, which is suitable for use in reading an image by means of an image sensor such as a line sensor, etc..
2. Description of the Related Art
Photoelectric output characteristics of respective pixels as photoelectric converting elements of an image sensor such as a line sensor, etc. are not necessarily linear over the entire range of exposure, as indicated by symbols A, B, C in FIG. 1. In particular, photoelectric output characteristics tend to vary increasingly as exposure approaches darkness levels. In addition, there is a specific current-on-darkness flow in the respective pixels even when there is no light incident thereon. An output voltage corresponding to the current-on-darkness appears as an output-on-darkness.
A method of correcting such variations in image signal photoelectric output characteristics has been disclosed in Japanese Laid-Open Patent Publication No. 62(1987)-202673, for example. According to the disclosure, a so-called shading correction is made at a portion where exposure increases so as to cause respective photoelectric output characteristics to converge. Then, a correction is made in such a manner that the respective photoelectric output characteristics are concentrated on a zero value at the point of the output-on-darkness, i.e., at the zero point of the exposure. The resulting corrected characteristics are represented by the symbols A', B' and C' in FIG. 2. The characteristic A' in FIG. 2 will be equivalent to an ideal linear characteristic.
However, the disclosure has a drawback in that the respective photoelectric output characteristics differ from each other at the range of a low exposure close to the level-on-darkness even when such a correction is made, thereby causing stripe-like density unevenness to appear in a sub-scanning direction of an output image.
An example of a technique of solving such a drawback is disclosed in Japanese Patent Application No. 63(1988)-246864. According to this method, respective values outputted from all the effective pixels of an image sensor are set equal to each other at a minimum value of an exposure range (hereinafter called "read range") required to read an image. These output values may also be set equal to each other at an exposure value X, which is smaller than the read range as shown by symbols A", B" and C" in FIG. 3.
However, this disclosure also has a drawback in that when the read range is determined in this way, the dynamic range relative to the read range is narrowed, in other words, the difference between the minimum value and the maximum value of the read range is reduced. Also, even when the read range is extended to the low exposure range, a problem arises in that the stripe-like density unevenness is visible.
Further, the disclosure has another drawback in that since an offset value, which varies according to variations in ambient temperature, exists in an image signal outputted from pixels of an image sensor such as a line sensor, step-wise density unevenness appears in the sub-scanning direction of the output image immediately after the power supply of an image reader is turned on where the correction processing relative to the offset value is not made.